Enhanced flip chip structure using copper column interconnect

ABSTRACT

A flip chip package includes: a carrier coupled to a die. The carrier includes: at least a via, for coupling the surface of the carrier to electrical traces in the carrier; and at least a capture pad electrically coupled to the via, wherein the capture pad is plated over the via. The die includes: at least a bond pad formed on the surface of the die; and at least a copper column, formed on the bond pad for coupling the die to the capture pad on the carrier, wherein part of the copper column overhangs the via opening.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.13/612,860, which was filed on Sep. 13, 2012, and claims the benefit ofU.S. Provisional Application No. 61/604,681, filed on Feb. 29, 2012.

BACKGROUND OF THE INVENTION

Flip chip technology is a method for coupling a chip (die) to a carrier,substrate or circuit board, wherein the die is electrically connected tothe carrier without using bond wires. Solder bumps on the die surfacedisposed over the bond pads are used as bonding means, and the chip isthen ‘flipped’ so that it is face down on the carrier. The solder bumpsenable electrical coupling to traces in the carrier by means of capturepads and vias. An epoxy covering then ‘under fills’ the structure toabsorb the stress. This technique allows for shorter interconnectlengths as well as more area available for routing.

In conventional technologies, the vias (which couple to electricaltraces in the carrier) are filled in with conductive material. Theresultant structure is usually not completely flat, such that a dimpleis formed on the surface of the conductive material at the opening ofthe via. The top of the carrier is then plated to form capture pads overeach via, where each capture pad is designed to have a similar diameterto the solder bumps on the die. The plating follows the contours of thefilled-in via, such that the capture pad also has a dimpled surface,allowing the solder bump to sit in the dimple.

Newer technologies replace the solder bumps with copper (Cu)pillars/columns having a small solder bump at one end for contacting thecapture pad. Please refer to FIG. 1A, which shows a diagram of aconventional die package 100 using solder bumps 73 to couple to vias 41,and FIG. 1B, which shows a conventional die package 150 using copper(Cu) columns 81 to couple to vias 41. In both diagrams, the samenumerals are used to denote the same components.

FIG. 1A shows a die package 100, comprising a die 112, which has aplurality of bond pads 28 on its surface, each bond pad 28 having asolder bump 73 formed thereon. As shown in the diagram, the die 112 isflipped to couple to a carrier 114 by means of the solder bumps 73. Thecarrier 114 has a plurality of capture pads 34 on its surface, eachcapture pad 34 being formed over a via 41. The vias 41 couple toelectrical traces (not shown) in the carrier 114. The vias 41 are filledwith conductive material, represented by the diagonal lines.

FIG. 1B shows a die package 150, comprising the die 112, flipped tocouple to the carrier 114. The bond pads 28 have copper columns 81formed thereon rather than solder bumps, for coupling the die 112 to thecarrier 114. As shown in the diagram, the copper columns 81 have asmaller diameter than the solder bumps 73 in FIG. 1A. In addition, eachcopper column 81 has a solder bump 93 formed at its end, the solderbumps 93 being of a similar diameter to the copper columns 81. The vias41 are filled in with conductive material, as in FIG. 1A. The capturepads 54 in FIG. 1B are of a smaller diameter than the capture pads 34 inFIG. 1A, corresponding to the smaller diameter of the copper columns 81.

As illustrated in the two diagrams, the capture pads 34, 54 are formedto have a similar diameter to the connecting solder bumps 73 and 93,respectively. When Copper columns 81 are utilized, their smallerdiameter as compared to the conventional solder bumps 73 means thecapture pads 54 can be formed with a similarly smaller diameter; the useof Copper columns 81 can therefore free up the bonding area. The smallerdiameter solder bump 93, however, has the disadvantage of having poorbonding contact with the plated capture pad 54; in particular, due tothe presence of the dimple. Increasing the diameter of the Coppercolumns 81 can improve the bump-dimple contact, but this involvesincreasing the cap size, which is not desirable, and also negates theincreased bonding area advantage.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide a flipchip package structure that utilizes copper columns for bonding means,wherein there is good bonding contact between the die and the carrier.This is achieved by providing copper columns wherein part of each coppercolumn overhangs a corresponding via opening, and the part of the coppercolumn that does not overhang the via opening contacts a correspondingcapture pad on one side of a corresponding via only, wherein the capturepads are plated to be asymmetrical about via openings in the carrier.

A flip chip package comprises: a carrier coupled to a die. The carriercomprises: at least a via, for coupling the surface of the carrier toelectrical traces in the carrier; and at least a capture padelectrically coupled to the via, wherein the capture pad is plated overthe via. The die comprises: at least a bond pad formed on the surface ofthe die; and at least a copper column, formed on the bond pad forcoupling the die to the capture pad on the carrier, wherein part of thecopper column overhangs the via opening.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram of a conventional flip chip structure where solderbumps disposed on the die couple to capture pads on the carrier.

FIG. 1B is a diagram of a conventional flip chip structure using coppercolumns on the die to couple to capture pads on the carrier.

FIG. 2 is a cross-sectional diagram of a flip chip structure accordingto an exemplary embodiment of the present invention.

FIG. 3A is a top view of a first embodiment of the flip chip structureshown in FIG. 2.

FIG. 3B is a top view of a second embodiment of the flip chip structureshown in FIG. 2.

FIG. 3C is a top view of a third embodiment of the flip chip structureshown in FIG. 2.

DETAILED DESCRIPTION

The present invention provides a novel structure for a flip chip packagethat utilizes copper columns for coupling a die to a carrier, whereinthere is good bonding contact between the die and the carrier, as wellas greater flexibility of the bonding structure.

In the following, the diagrams and accompanying descriptions will referto preferred exemplary embodiments; however, one skilled in the art willbe able to perform appropriate modifications after reading the followingdisclosure. It will be appreciated that any modifications to theproposed design which follow the same inventive concepts as those laidout in the disclosure also fall within the scope of the invention.

Please refer to FIG. 2, which is a cross-sectional diagram of a proposedflip chip structure 200 according to an exemplary embodiment of thepresent invention, and FIGS. 3A, 3B and 3C, which are top views of thestructure 200 shown in FIG. 2 corresponding to different respectiveembodiments. Where elements shown in FIGS. 2, 3A, 3B and 3C have thesame structure and function as elements shown in FIGS. 1A and 1B, thesame numerals have been used.

The proposed flip chip package 200 consists of a die 112 coupled to acarrier 114 in the flip chip manner, and coupled by the means of coppercolumns 230. Each copper column 230 has a small solder bump 232 on theend for contacting capture pads 251 formed on the surface of the carrier114. The carrier 114 also has a number of vias 41 for coupling thecapture pads 251 to traces (not shown) in the carrier 114.

In FIG. 2, part of the copper column 230 (and the solder bump 232 on theend of the copper column 230) overhangs the vias 41. The other part ofthe copper column 230 (and the solder bump 232 on the end of the column230) is placed on the capture pad 251 so that the copper columns 230 areplaced on only one side of their corresponding capture pads 251, asillustrated in FIGS. 3A, 3B and 3C. This ensures both the bestelectrical connectivity and the best mechanical stability.

In order to ensure the electrical connectivity, three differentembodiments are disclosed herein, which are respectively illustrated inFIGS. 3A, 3B and 3C. As can be seen by comparing these diagrams withFIG. 2, the cross-sectional appearance of the copper columns 230 andcapture pads 251 is the same, but the top views as illustrated in FIGS.3A, 3B and 3C are different from each other. The respective differenceswill be detailed in the following, with reference to their accompanyingdiagrams.

Please refer to FIG. 3A, which illustrates a top view of the capturepads 251, vias 41 and solder bumps 232 as formed on the carrier 114according to a first embodiment. As shown in this embodiment, thecapture pads 251 are symmetrical about the vias 41 apart from onerectangular section which extends out to one side of the capture pads251. The copper columns 230 and the solder bumps 232 are disposed partlyon this rectangular section, with the other part overhanging the vias41.

Please refer to FIG. 3B, which illustrates a top view of the capturepads 251, vias 41 and solder bumps 232 as formed on the carrier 114according to a second embodiment. As shown in this embodiment, thecapture pads 251 are asymmetrical about the vias 41, and the part of thecopper columns 230 and solder bumps 232 which contact the capture pads251 are disposed on the side of the capture pads 251 having the greaterarea.

Please refer to FIG. 3C, which illustrates a top view of the capturepads 251, vias 41 and solder bumps 232 as formed on the carrier 114according to a third embodiment. As shown in this embodiment, thecapture pads 251 are asymmetrical about the vias 41, and the part of thecopper columns 230 and solder bumps 232 which contact the capture pads251 are disposed on the side of the capture pads 251 having the greaterarea, as in the previous embodiment. The difference between the secondand third embodiment is that the copper columns 230 and solder bumps 232are shaped to follow the shape of the capture pads 251. In FIG. 3C,solder bumps 232 (and therefore copper columns 230) having kidney-shapedor C-shaped cross-sectional areas are formed.

As detailed above and illustrated in FIGS. 3A, 3B and 3C, the coppercolumns 230 will only contact one side of the capture pads 251 about thevias 41. This structure allows the copper column 230 to take maximumadvantage of the capture pad 251 conductivity, and ensures the coppercolumn 230 will have good bonding contact.

The shape of the capture pad 251 is also not limited to the egg-shapedand ellipse-shaped examples detailed above. In general, it is desirableto have a capture pad shape that does not require a large amount ofextra material to be added. As a typical die package will have aplurality of vias (and therefore capture pads) disposed thereon, it isalso desirable that the shape of the capture pads allow as many capturepads as possible to be disposed on the carrier surface, to allow greaterbonding possibilities. The specific number and individual shape ofcapture pads can be according to a designer's requirements.

As well as freeing up the bonding area of the die package, the third,fourth and fifth embodiments which disclose asymmetrical capture padsand copper columns that follow the shapes of the capture pads have theadded advantage of reducing stress in the Extra Low K (ELK) layers. Dueto the copper columns being shaped to be asymmetrical, the coppercolumn—capture pad bonds will be more stable than the bonds between thecopper columns and the capture pads of the conventional art. In theexamples shown in FIGS. 3C, 4A and 4B, the kidney or C-shaped coppercolumn 230 has less potential for movement about a central point thanthe conventional copper columns 81, so there is less potential for ELKlayer cracks to occur than in the conventional methods. Therefore, theproposed structure not only improves the electrical connectivity of thedie package but also improves its mechanical stability.

By keeping the copper column very close to the via so that part of thecopper column overhangs the via, maximum contact can be ensured, whileminimizing the amount of material that needs to be used for the capturepad. If the capture pad is kept below a certain diameter, then morespace on the carrier is available for bonding.

As will be appreciated by one skilled in the art, the shape of thecopper column and corresponding shape of the capture pad are not limitedto those examples provided in the disclosure. Any shaped copper columnthat follows an alternative (non-conventional) shape of a capture padfor bonding purposes falls within the scope of the invention.

In summary, the disclosure details a flip chip package, wherein coppercolumns are used to contact capture pads, which in turn are coupled tovias in the carrier that are coupled to circuit traces within thecarrier. Part of the copper columns overhang the corresponding vias, andthe capture pads may be asymmetrical about a centre of a respective via.In one embodiment, the copper column is shaped to follow the perimeterof the capture pad. This structure not only frees up the amount ofbonding space available on a surface of the chip, but also results ingreater stability due to less stress being placed on the ELK layers,thereby resulting in a structure with high electrical connectivity andmechanical stability.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A flip chip package comprising: a carrier,comprising: at least a via, for coupling the surface of the carrier toelectrical traces in the carrier, wherein the via is open and unfilled;and at least a capture pad electrically coupled to the via, wherein thecapture pad is plated over the via; and a die, coupled to the carrier,comprising: at least a bond pad formed on the surface of the die; and atleast a copper column, formed on the bond pad for coupling the die tothe capture pad on the carrier, wherein part of the copper columnoverhangs the via opening and part of the copper column does notoverhang the via opening, so that only part of the via opening iscovered by the copper column.
 2. The flip chip package of claim 1,wherein the capture pad is asymmetrical about at least one axis runningthrough the centre of the via opening, and the part of the copper columnwhich does not overhang the via opening is disposed on the side of thecapture pad having a greater surface area.
 3. The flip chip package ofclaim 2, wherein the copper column is curved about the vertical planesuch that it has at least one axis of asymmetry.
 4. The flip chippackage of claim 3, wherein the capture pad is egg shaped and a top viewof the copper column is C-shaped.
 5. The flip chip package of claim 2,wherein the capture pad comprises a rectangular section which is formedon one side about the via opening, and the part of the copper columnwhich does not overhang the via opening copper column is disposed on therectangular section.
 6. The flip chip package of claim 1, wherein thecopper column overhangs the bond pad.
 7. The flip chip package of claim2, wherein at least one side of the copper column is parallel to an edgeof the bond pad.